The present invention relates to an electroless plating method. More particularly, it relates to a technique for forming plating catalyst metal nuclei on a substrate for electroless plating.
An electroless plating method is relatively simple as a method for forming a metal conductive coating layer and suitable for mass production, and if necessary, electrolytic plating may be applied after the electroless plating. Thus, electroless plating is widely used in various fields for functional and decorative applications.
Further, in recent years, high densification is required for printed circuits reflecting an increasing tendency for small sizes of electric and electronic devices, and printed circuit boards have been developed in favour of through hole substrates or mass lamination substrates.
The plating of through holes has been conducted by a panel plating method or a pattern plating method by means of electrolytic plating. By such a method, however, the process is cumbersome. In a case where the aspect ratio is high as in the case of the through holes of a mass lamination substrate, an attention has been drawn to an electroless plating method in order to improve the uniformity of the conductive coating layer in the through holes, and electroless plating has been practically employed to some extent.
By the applicaion of electroless plating to plastic materials, it is possible not only to improve the heat resistance, rigidity and surface hardness of the plastic materials, but also to impart a metallic gloss and metallic properties to the surface of the plastic materials. Therefore, such electroless plating is preferably used for the production of home electrical appliances, interior or exterior parts of automobiles, office appliances and general industrial appliances.
Printed circuit boards rapidly developed in recent years, are usually prepared by a process wherein a copper foil formed on a paper substrate, on a thermoset plastic substrate made of e.g. a glass fiber-reinforced phenol resin or epoxy resin, or on a flexible substrate made of e.g. polyimide, polyester or polysulfone, is removed at unnecessary portions. An attention has been drawn to an electroless plating method with a view to simplification of the process and improvement of the circuit properties.
Further, the electroless plating method is expected to be further developed for a so-called electro magnetic shielding technique whereby electric conductivity is imparted to the casings for electronic devices to eliminate erroneous operation of a degital circuit due to electromagnetic waves, since the resulting metal conductive coating layer is extremely thin, light in weight and excellent in the shielding effect, as compared with other methods for providing conductivity, such as a metal flame spraying method or an electro-conductive painting coating method.
The electroless plating method usually comprises a surface treatment step such as degreasing of the substrate, a surface roughening step to improve the adhesive strength of the plated layer, a step of forming plating catalyst metal nuclei, a step of washing with water and an electroless plating step.
In the step of forming plating catalyst metal nuclei, the substrate is dipped in a solution of an activating agent. The activating agent solution may be an aqueous stannous salt solution or an aqueous palladium chloride solution in a case where the substrate is a non-conductive material such as a plastic material. It may be an acidic aqueous solution containing stannous chloride and palladium chloride in a case where the substrate has non-conductive and conductive surfaces.
When it is desired to partially apply electroless plating as in the case of an additive method printed circuit board, only the desired portions will be activated. As such a method, there is a method wherein an ink composed of a resin adhesive containing a metal such as nickel or palladium, or a salt thereof, is coated on predetermined portions of a substrate, followed by dipping in a plating bath for plating, or a method as disclosed in Japanese Unexamined Patent Publication No. 88891/1984 (U.S. Pat. No. 4,368,281) wherein an ink composed of a resin adhesive containing an organic metal complex of e.g. nickel, palladium or platinum, is coated on a substrate, followed by heating or dipping in a specific reducing agent, and then by plating.
Further, there is a method wherein the entire surface of a substrate is activated, and the portions of the substrate where no plating is required, are covered with a plating resist, follwod by electroless plating, and then by the removal of the plating resist.
When a conductive coating layer is to be formed in through holes solely by electroless plating, plating catalyst metal nuclei will be deposited by a dipping method.
Namely, for the preparation of a circuit board, plating catalyst metal nuclei are deposited on the entire surface of a copper-clad laminate, then the necessary circuit portions are masked with a dry film resist, and the unnecessary copper foil is removed by etching, followed by electroless plating.
In the conventional electroless plating method, the activating agent for forming plating catalyst metal nuclei is in the form of a solution, and a substrate is dipped therein, whereby a special rack or the like is required, and it is difficult to make this step a continuous process.
Further, when a conductive coating layer in the through holes is to be formed solely by electroless plating, palting catalyst metal nuclei will be formed by a dipping method, whereby it is difficult to make the step continuous.
When an activating agent such as a combination of an aqueous stannous salt solution and an aqueous palladium chloride solution, or an acidic aqueous solution comprising stannous chloride and palladium chloride, is used, tin ions are like to remain on the substrate surface, which adversely affect the physical properties of the metal conductive coating layer formed by electroless plating.
In the method where an ink composed of an adhesive resin containing a metal such as nickel or palladium, or salt thereof, or an ink disclosed in Japanese Unexamined Patent Publication No. 88891/1984 (U.S. Pat. No. 4,368,281) is used to activate only the desired portions in order to apply electroless plating only to the desired portions, a metal conductive coating layer is formed on an adhesive resin, whereby the inherent properties of the substrate itself are likely to be impaired.
In the method wherein electroless plating is applied while covering the portions of the substrate where no plating is required, with a plating resist, an expensive palladium metal in the activating agent is deposited on the portions where no plating is required, such being uneconomical, and cumbersome steps such as the coating and peeling of the plating resist are required. When such a method is applied for the production of a printed circuit board, palladium metal is likely to remain between circuits, whereby the dependability in e.g the withstand voltage of the circuits tends to be poor.
The method in which the dry film resist is employed, has a drawback that the dry film resist is expensive, and a high level of technique is required for the step of masking the necessary circuit portions, and this method is not useful for landless through holes (bia holes) which are commonly employed in recent years.